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NetBSD 6.1.5 - man page for mount_union (netbsd section 8)

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MOUNT_UNION(8)			   BSD System Manager's Manual			   MOUNT_UNION(8)

NAME
     mount_union -- mount union filesystems

SYNOPSIS
     mount_union [-b] [-o options] directory uniondir

DESCRIPTION
     The mount_union command attaches directory above uniondir in such a way that the contents of
     both directory trees remain visible.  By default, directory becomes the upper layer and
     uniondir becomes the lower layer.

     Both directory and uniondir are converted to absolute paths before use.

     The options are as follows:

     -b      Invert the default position, so that directory becomes the lower layer and uniondir
	     becomes the upper layer.  However, uniondir remains the mount point.

     -o      Options are specified with a -o flag followed by a comma separated string of
	     options.  See the mount(8) man page for possible options and their meanings.

     Filenames are looked up in the upper layer and then in the lower layer.  If a directory is
     found in the lower layer, and there is no entry in the upper layer, then a shadow directory
     will be created in the upper layer.  It will be owned by the user who originally did the
     union mount, with mode "rwxrwxrwx" (0777) modified by the umask in effect at that time.

     If a file exists in the upper layer then there is no way to access a file with the same name
     in the lower layer.  If necessary, a combination of loopback and union mounts can be made
     which will still allow the lower files to be accessed by a different pathname.

     Except in the case of a directory, access to an object is granted via the normal filesystem
     access checks.  For directories, the current user must have access to both the upper and
     lower directories (should they both exist).

     Requests to create or modify objects in uniondir are passed to the upper layer with the
     exception of a few special cases.	An attempt to open for writing a file which exists in the
     lower layer causes a copy of the entire file to be made to the upper layer, and then for the
     upper layer copy to be opened.  Similarly, an attempt to truncate a lower layer file to zero
     length causes an empty file to be created in the upper layer.  Any other operation which
     would ultimately require modification to the lower layer fails with EROFS.

     The union filesystem manipulates the namespace, rather than individual filesystems.  The
     union operation applies recursively down the directory tree now rooted at uniondir.  Thus
     any filesystems which are mounted under uniondir will take part in the union operation.
     This differs from the union option to mount(8) which only applies the union operation to the
     mount point itself, and then only for lookups.

EXAMPLES
     The commands

	   mount -t cd9660 -o ro /dev/cd0a /usr/src
	   mount -t union /var/obj /usr/src

     mount the CD-ROM drive /dev/cd0a on /usr/src and then attaches /var/obj on top.  For most
     purposes the effect of this is to make the source tree appear writable even though it is
     stored on a CD-ROM.

     The command

	   mount -t union -o -b /sys $HOME/sys

     attaches the system source tree below the sys directory in the user's home directory.  This
     allows individual users to make private changes to the source, and build new kernels, with-
     out those changes becoming visible to other users.  Note that the files in the lower layer
     remain accessible via /sys.

SEE ALSO
     intro(2), mount(2), unmount(2), fstab(5), fsck_ffs(8), mount(8), mount_null(8), sysctl(8)

HISTORY
     The mount_union command first appeared in 4.4BSD.

BUGS
     Without whiteout support from the filesystem backing the upper layer, there is no way that
     delete and rename operations on lower layer objects can be done.  An attempt to mount a
     union directory under one which does not have whiteout support will return EOPNOTSUPP
     ("Operation not supported").  Whiteout support can be added to an existing FFS filesystem by
     using the -c option of fsck_ffs(8).

     Running find(1) over a union tree has the side-effect of creating a tree of shadow directo-
     ries in the upper layer.

BSD					 February 5, 2008				      BSD
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